This invention relates to a spark plug employed in an internal combustion engine. More specifically, the present invention relates to an electrode material of a spark plug and its composition which satisfy required fundamental performances and can improve heat resistance, and therefore can be applied to a high-performance spark plug employed in a high-performance or high-advanced engine subjected to severe thermal load environment having not been experienced by conventional engines.
A spark plug conventionally employed in an internal combustion engine of an automotive vehicle, as understood with reference to FIG. 1, comprises a center electrode 30 fixed to an insulator 20 and a ground electrode 40 welded to 51 a metal housing 10. The metal housing 10 firmly surrounds an outer surface of insulator 20. The spark plug is securely fixed to an engine body via the metal housing 10. A distal end surface 41 of ground electrode 40 is opposed to an apical surface 31 of center electrode 30 so as to form a discharge gap 50. The discharge gap 50 causes a spark to ignite fuel gas mixture.
In this case, the material constituting an electrode material is required to satisfy sufficient high-temperature strength, anti-fusion property, high-temperature corrosion resistance, and spark exhaustion durability. For example, as preferable electrode material, U.S. Pat. No. 4,329,174 (corresponding to JP 60-43897) discloses a Ni-based alloy containing, in weight percentage (hereinafter, xe2x80x98%xe2x80x99 represents xe2x80x98weight %xe2x80x99), 0.2xcx9c3% Si, not larger than 0.5% Mn, and at least two kinds of additive components selected from the group consisting of 0.2xcx9c3% Cr, 0.2xcx9c3% Al, and 0.01xcx9c1% Y in addition to the main component Ni and unavoidable impurities.
However, recent internal combustion engines are required to operate at higher engine speeds. The recent combustion engines often use high-octane gasoline. These factors lead to the remarkable increase of combustion temperature in the combustion chamber. Accordingly, the spark plug electrode material, constituting the center electrode 30 and the ground electrode 40, is inevitably subjected to such high-temperature combustion atmosphere. The above-described conventional Ni-base alloy shows appropriate anti-fusion property, high-temperature corrosion resistance, and spark exhaustion durability in the high-temperature atmosphere. However, the above-described conventional Ni-base alloy is dissatisfactory in high-temperature strength. Thus, the life of the above-described conventional Ni-base alloy is relatively short.
To satisfy such requirements, Japanese Patent No. 2587864 discloses a Ni-base alloy containing Si, Mn, Cr and Al in addition to Ni, unavoidable impurities, and appropriate amount of rare earth elements. Adding the rare earth elements is effective to improve the high-temperature strength. More specifically, adding a very few amount of Ce, Nd, or La to an electrode material leads to remarkable improvement of high-temperature strength in a combustion atmosphere of 800xc2x0 C., according to the disclosure of this patent.
However, the recent lean-burn combustion technique realized by a direct fuel injection system or the like necessitates many of the automotive manufactures to develop a high output/high performance and clean engine which is excellent in fuel consumption due to reduction of idling speed and is also capable of reducing the amount of CO2 or other harmful emission gases. To realize such advanced engines, the recent spark plugs are required to have excellent heat resistance in a severer high-temperature combustion atmosphere, e.g., 950xc2x0 C. or above at the ground electrode constituting the spark plug.
However, the excellent anti-fusion property, high-temperature corrosion resistance, and spark exhaustion durability of the above-described conventional electrode material is limited to the temperature level of approximately 800xc2x0 C. If the above-described conventional electrode material is exposed to a severe combustion atmosphere exceeding 950xc2x0 C., the ground electrode material will cause a damage accompanied by abnormal oxidation in the grain boundaries. The discharge gap will increase to a greater value (e.g., 1.2 mm) from its initial value (e.g., 0.8 mm). An increased amount of voltage will be required to ignite the spark plug. The spark plug may cause misfire in the worst case. In this manner, the above-described conventional electrode material will cause various problems when it is employed in a spark plug for an advanced high output/high performance engine.
In view of the above-described problems encountered in the prior art, the present invention has an object to provide a spark plug for an internal combustion engine which satisfies required fundamental performances of the spark plug electrode material and assures excellent heat resistance in a severe combustion atmosphere exceeding 950xc2x0 C. which was not experienced by the conventional engine.
To accomplish the above and other related objects, the inventors of this application have worked on the research and development focused into the electrode materials which satisfy the required fundamental performances of an engine spark plug and have excellent heat resistance in a severer high-temperature combustion environment. As a result of research and development conducted by the inventors of this application, the optimized composition and size for an electrode material are experimentally found out. The present invention is derived from the experimental result.
The present invention provides a first spark plug for an internal combustion engine comprising an insulator, a center electrode fixed to a leg portion of the insulator which is exposed to a combustion chamber of an internal combustion engine, a metal housing firmly surrounding an outer surface of the insulator, and a ground electrode fixed to an end of the metal housing so as to form a spark discharge gap between the center electrode and the ground electrode.
The first spark plug of the present invention is characterized in that at least one of the center electrode and the ground electrode is a Ni-base alloy containing, in weight percentage, 0.5xcx9c2.5% Si, 0.1xcx9c1.2% Mn, 3.2xcx9c5.0% Al, 0.9xcx9c2.8% Cr, 0.001xcx9c0.025% C in addition to Ni and unavoidable impurities, and a value S/V is in a range from 1.7 mmxe2x88x921 to 3.9 mmxe2x88x921 when xe2x80x98Sxe2x80x99 represents a surface area of the ground electrode and xe2x80x98Vxe2x80x99 represents a volume of the ground electrode.
When the electrode material has the above-described composition, it becomes possible to provide a spark plug which satisfies the fundamental performances required for an internal combustion engine spark plug and assures reliable heat resistance even in a severe combustion atmosphere exceeding 950xc2x0 C. in electrode temperature.
Furthermore, when the ratio S/V of the surface area xe2x80x98Sxe2x80x99 to the volume xe2x80x98Vxe2x80x99 of the ground electrode is in the range from 1.7 mmxe2x88x921 to 3.9 mmxe2x88x921, not only the heat resistance can be assured in the combustion atmosphere exceeding 950xc2x0 C. in electrode temperature but also the bending work of the ground electrode can be facilitated. If the ratio S/V is less than 1.7 mmxe2x88x921, the bending work of the ground electrode for adjusting an initial discharge gap will become difficult. If the ratio S/V is larger than 3.9 mmxe2x88x921, the thermal conductivity of the ground electrode will be worsened and it will be difficult to obtain reliable heat resistance.
According to the present invention, it is preferable that at least one of the center electrode and the ground electrode is a Ni-base alloy containing, in weight percentage, 1.0xcx9c2.5% Si, 0.1xcx9c0.9% Mn, 3.5xcx9c5.0% Al, 1.3xcx9c2.5% Cr, 0.001xcx9c0.025% C in addition to Ni and unavoidable impurities.
When the electrode material has the above-described composition, it becomes possible to provide a spark plug which satisfies the fundamental performances required for an internal combustion engine spark plug and assures excellent heat resistance even in a severer combustion atmosphere exceeding 1,050xc2x0 C. in electrode temperature.
Furthermore, according to the present invention, it is preferable that the ground electrode value S/V is in a range from 1.7 mmxe2x88x921 to 3.0 mmxe2x88x921. This is effective to assure excellent heat resistance in the severer combustion atmosphere exceeding 1,050xc2x0 C. in electrode temperature. The bending work of the ground electrode can be further facilitated. The reason why the ground electrode value S/V is set in a range from 1.7 mmxe2x88x921 to 3.0 mmxe2x88x921 is substantially explained in the above description.
The present invention provides a second spark plug for an internal combustion engine comprising an insulator, a center electrode fixed to a leg portion of the insulator which is exposed to a combustion chamber of an internal combustion engine, a metal housing firmly surrounding an outer surface of the insulator, and a ground electrode fixed to an end of the metal housing so as to form a spark discharge gap between the center electrode and the ground electrode. The second spark plug of the present invention is characterized in that at least one of the center electrode and the ground electrode is constituted by a base material which forms a surficial aluminum oxide when it is left in an atmospheric environment at a temperature equal to or higher than 950xc2x0 C. for a duration equal to or longer than 50 hours.
When the spark plug of this invention is used in the high-temperature environment exceeding 950xc2x0 C., the surficial aluminum oxide is stably formed on the electrode base material. The surficial aluminum oxide effectively protects the inside portion of the electrode base material against oxidation. When a tip (i.e., a discharge member) is welded on the center electrode or the ground electrode serving as the electrode base material, the surficial aluminum oxide effectively protects the bonded boundary between the tip and the electrode base material against oxidation. Accordingly, the present invention provides an excellent spark plug which is capable of preventing the electrode base material from abnormally oxidizing, preventing the tip from falling off the electrode base material due to oxidation in the bonded boundary, and assuring long-lasting high performance, even in a very severe thermal load environment.
According to the present invention, it is preferable that the surficial aluminum oxide is stably formed as an oxide coating layer densely covering the electrode base material. Thus, the surficial aluminum oxide surely prevents the oxygen ions from diffusing inside the electrode base material. The effect of suppressing the oxidation is further enhanced.
According to the present invention, it is preferable that a portion of the ground electrode having not been subjected to bending deformation has a hardness Hv (0.5) equal to or less than 210 when the hardness is measured with a testing force of 4.903N according to a micro Vickers"" hardness testing method regulated in JIS standard Z2244.
In general, adding Al in the electrode base material worsens the bending workability due to increase of hardness. However, when the hardness Hv (0.5) of the ground electrode is equal to or less than 210, it becomes possible to adequately suppress the springback into a practically allowable range when the ground electrode is subjected to bending deformation to form a discharge gap. Accordingly, the discharge gap can be accurately formed.
According to the present invention, the bending workability can be further improved. The accuracy in forming the discharge gap can be further improved.
Furthermore, according to the present invention, at least one of the center electrode and the ground electrode may serve as a base material. A tip, being made of a noble metal or its alloy, is fixed to a surface of the base material by welding.
When the noble metal tip serving as a discharge member is securely welded to the electrode base material, not only the spark exhaustion durability is greatly improved but also the bonding reliability of the noble metal tip welded to the electrode material can be greatly improved. For example, the electrode material preferably used in this case is a so-called NCF 600 (composition: Cr=15.5%, Fe=7%, C less than 0.15%, Mn less than 1%, Si less than 0.5%, and the remainder=Ni+unavoidable impurities). The composition of the present invention is different from composition of NCF 600. Having the composition defined by the present invention makes it possible to reduce the amount of Cr, thereby suppressing Cr from depositing into the bonded surface. Furthermore, adding Al according to the present invention is effective to protect the inside portion of the electrode base material against oxidation. This surely prevents generation of cracks caused by a thermal stress and also prevents oxidation of the bonded boundary. Accordingly, in a very severe thermal load environment, it is possible to assure appropriate heat resistance and also obtain excellent spark exhaustion durability and bonding reliability.
According to the present invention, it is preferable that the tip is made of a Pt alloy including not less than 50 weight % Pt as a chief component and at least one additive component selected from the group consisting of Ir, Rh, Ni, W, Pd, Ru, Os, Y, and Y2O3. On the other hand, according to the present invention, it is preferable that the tip is made of an Ir alloy including not less than 50 weight % Ir as a chief component and at least one additive component selected from the group consisting of Pt, Rh, Ni, W, Pd, Ru, Os, Y, and Y2O3.
When the tip is made of the above-described material, it becomes possible to improve the spark exhaustion durability. Even when the tip is used in an engine subjected to a large thermal load, it is possible to assure a satisfactory life of the spark plug.
According to the present invention, it is preferable that the ground electrode has a plated layer formed on a surface thereof.
In general, a spark plug may be left in a high-temperature and high-humid atmosphere before it is installed in an internal combustion engine. However, according to the spark plug of this invention, the plated layer formed on the surface of the ground electrode brings preferable functions and effects when the spark plug is installed in the internal combustion engine. Forming the plated layer on the ground electrode improves the appearance and the commercial value of a spark plug.
Like the above-described NCF 600, the electrode material having good heat resistance usually comprises a large amount of Cr and Fe additives and therefore tends to form a thick oxide film on the electrode surface. It is therefore difficult to assure satisfactory plating adhesion properties. The plated layer may peel off the electrode material, when the ground electrode is subjected to bending work. To solve this problem, it is generally necessary to apply a masking in the plating process. This increases the manufacturing costs and deteriorates the product quality in appearance.
On the other hand, the spark plug of the present invention having the composition of the present invention has a small amount of Cr and contains no Fe. Thus, the present invention brings satisfactory plating adhesion properties. The present invention provides a spark plug electrode material having preferable functions and effects durable in a very severe thermal load environment. Furthermore, the present invention brings the effects of reducing the manufacturing costs and improving the product quality in appearance.
The following is the reason why the present invention strictly defines the composition of a Ni-base alloy constituting the ground electrode of a spark plug. In the following explanation, all of the values expressed by xe2x80x98%xe2x80x99 are the ones by the weight percent. A first aspect of the present invention defines an optimum range of the Ni-base alloy composition which is preferable for assuring the heat resistance in the combustion atmosphere exceeding 950xc2x0. A second aspect of the present invention defines an optimum range of the Ni-base alloy composition which is preferable for assuring the heat resistance in the combustion atmosphere exceeding 1,050xc2x0.
(a) Si
Si component has a function of improving the high-temperature corrosion resistance as well as the spark exhaustion durability. However, such preferable effects will not be satisfactorily obtained when the content of Si is less than 0.5%. On the other hand, when the content of Si exceeds 2.5%, working cracks may be produced in an electrode material during its manufacturing process. Accordingly, the present invention defines a range from 0.5 to 2.5% as a preferable content of Si (refer to the first aspect of the present invention). Furthermore, to assure the preferable effects of Si even in a further higher temperature combustion atmosphere, the present invention defines a range from 1.0 to 2.5% as a preferable content of Si (refer to the second aspect of the present invention).
(b) Mn
Mn component is an indispensable component due to its deoxidizing and desulfurizing functions required in the ingot process. However, such preferable functions will not be satisfactorily obtained when the content of Mn is less than 0.1%. On the other hand, when the content of Mn exceeds 1.2%, the high-temperature corrosion resistance will deteriorate greatly. Accordingly, the present invention defines a range from 0.1 to 1.2% as a preferable content of Mn (refer to the first aspect of the present invention). Furthermore, to assure the preferable effects of Mn even in a further higher temperature combustion atmosphere, the present invention defines a range from 0.1 to 0.9% as a preferable content of Mn (refer to the second aspect of the present invention).
(c) Al
Al component forms a dense oxide protective coating layer on an electrode surface when the electrode temperature exceeds 950xc2x0 C. Al component has a function of suppressing the oxidation in the grain boundaries, thereby improving the high-temperature corrosion resistance and the high-temperature strength. However, such preferable functions will not be satisfactorily obtained when the content of Al is less than 3.2%. On the other hand, when the content of Al exceeds 5.0%, the workability will deteriorate. Accordingly, the present invention defines a range from 3.2 to 5.0% as a preferable content of Al (refer to the first aspect of the present invention). Furthermore, to assure the preferable effects of Al even in a further higher temperature combustion atmosphere, the present invention defines a range from 3.5 to 5.0% as a preferable content of Al (refer to the second aspect of the present invention).
(d) Cr
Cr component has a function of improving the high-temperature corrosion resistance. However, such preferable functions will not be satisfactorily obtained when the content of Cr is less than 0.9%. On the other hand, when the content of Cr exceeds 2.8%, the anti-fusion property will deteriorate. Accordingly, the present invention defines a range from 0.9 to 2.8% as a preferable content of Cr (refer to the first aspect of the present invention). Furthermore, to assure the preferable effects of Cr even in a further higher temperature combustion atmosphere, the present invention defines a range from 1.3 to 2.5% as a preferable content of Cr (refer to the second aspect of the present invention).
(e) C
C component has a deoxidizing function. Furthermore, C component forms a carbide which effectively suppresses excessive growth of crystal grains during the operation of the spark plug. Accordingly, adding the C component is appropriate if required. However, such preferable functions will not be satisfactorily obtained when the content of C is less than 0.001%. On the other hand, when the content of C exceeds 0.025%, the bending workability will deteriorate. Accordingly, the present invention defines a range from 0.001 to 0.025% as a preferable content of C.